<style>
  custom-checkbox-element {
    display: block;
    width: 20px;
    height: 20px;
  }
</style>
<div></div>
<custom-checkbox-element></custom-checkbox-element>
<script>
  customElements.define('custom-checkbox-element',
    class extends HTMLElement {
      constructor() {
        super();
        this.attachShadow({ mode: 'open' }).innerHTML = `
          <div><input/><input type="button"/></div>
          `;
      }
    }
  );
</script>
